Unit 2 at Pacific Gas and Electric Company's Diablo Canyon Power Plant returned to full power on Tuesday.
The unit safely went offline last week after an electrical disturbance.
An investigation revealed a flashover event happened during routine maintenance that involved pressure washing also known as "hot washing" of electrical insulators near the lightning arrester.
Sensing the disturbance, unit 2 automatically went offline to protect the equipment. Unit 1 was not affected.